This invention relates to a method of manufacturing reinforced articles of softened polyvinyl chloride. In particular, an assembly formed by a layer of reinforcement material having on each side at least one foil of softened polyvinyl chloride is introduced into a high-frequency field, and the parts of the foils situated along the edges of the reinforcement material are sealed together to form seams.
The reinforcement material is plate-shaped, may consist of several parts and generally has fibrous structure. For example, the reinforcement material may be composed of paper, cardboard or a plastic such as synthetic foam, cheap, moisture-sensitive but reasonably rigid cardboard being frequently used as a reinforcement material in articles made of softened polyvinyl chloride.
According to a conventional method of manufacturing such articles, one or several parts or sheets of reinforcement material are laid between foils of softened polyvinyl chloride. This assembly, which has a sandwich structure, is then placed in a high-frequency sealing machine in which the sheets of reinforcement material situated between the foils are sealed-in along the edges by means of a high-frequency electric field. For this purpose, the polyvinyl chloride foils must extend beyond the peripheral edges of the reinforcement material so as to enable the sealing together of the polyvinyl chloride foils along the edges. When several sheets of reinforcement material are used, some space or clearance is present between the various sheets.
The resulting flat sandwich product with sealed-in parts or sheets of reinforcement material, sometimes termed plano product, is then mechanically given the desired shape or configuration determined by the final product, for example, by folding.
The kind and quality of high-frequency field generated in the sealing machine is determined by the electrodes used therein. By varying the shape of the electrodes, the sealed seams obtained in the plano product can be varied. This means that the shape of the sealed-in parts of reinforcement material may differ considerably in accordance with the electrodes used so that a variety of articles can be manufactured from the plano product. Examples of such articles are packaging or storage units; for example, portfolios, cases, handbags, boxes, suitcases and the like.
After the shaping of the articles, further mechanical components of metal or synthetic resin such as hinges, closures, rings or clamps are nearly always provided. These components are fixed in the article in question by means of rivets or staples, the rivet or staple perforating the assembly of foils with intermediate reinforcement material.
The disadvantage of the method described is that air can penetrate into the various sealed-in compartments via the perforations caused by the staples and rivets so that the foil of softened polyvinyl chloride surrounding the reinforcement material starts showing ruffles. In the absence of perforations, the polyvinyl chloride foils will also show undulations after a shorter or longer period of time as a result of heating, for example, by sunlight, so that the product becomes unsalable.
According to another method, each foil is provided on one side over a portion of its surface with a layer of aqueous glue. The size and shape of any portion of the surface which is provided with the layer of glue must accurately correspond to the size and shape of the sheet of reinforcement material used. The sheets of reinforcement material are accurately disposed on the surface of one of the two foils provided with glue in such manner that the glued portions of the foil are exactly covered by the reinforcement material. The second foil is then provided in the same accurate manner over the surface of the reinforcement material remote from the first foil, with the layer of glue facing the reinforcement material. The portions of the two foils not provided with glue are then sealed together along the edges of the reinforcement material in a high-frequency field. The whole process is carried out in an expensive automatic glue-sealing machine which is designed especially for this purpose.
The advantage of this method over the first-mentioned method is that ruffles of the foils are substantially suppressed, even if comparatively thin foils of softened polyvinyl chloride are used.
A disadvantage of the method is the high purchasing price of the required automatic glue-sealing machine. A further disadvantage is that, as already described above, the layer of glue has to be provided very accurately. If the layer of glue extends to just beyond the reinforcement material, an imperfect seal is obtained; that is, a sealed seam having too low a sealing strength so that the seal can easily be broken. If, on the other hand, the layer of glue does not extend entirely up to the edges of the reinforcement material, an air-filled space will be present between the sealed seam and the foils glued to the reinforcement material after the sealing process. As a result, ruffles will be formed in the polyvinyl chloride foils under the influence of pressure and temperature variations of the ambient air and furthermore delamination of the glued foils may take place. Another disadvantage of this method is that when switching from one product to another, the operations required for readjustment of the automatic glue-sealing machine are time-consuming and in addition can be carried out only by highly skilled technical personnel. Consequently, the method described is less suitable for manufacturing products in comparatively small quantities.